It's unlikely that the storage was tampered with. More likely is that it isn't an s20 at all, but a fake with low-end internals (including limited storage) that has been programmed to report that it has more storage than it really has. Which "internet site" did you buy it from, and if it isn't too indelicate a question, was it cheaper than you might expect?
The other possibility is that it's a real s20 but faulty, but as the system storage and user storage are the same chip it feels a little unlikely that there could be a fault in the storage device that left all of the system software untouched and functional but did this.
Could you post a screenshot of the system information. Particularly the "baseband version": fakes are usually programmed to lie about what type of device they are, what software build they are running, etc, but the baseband (radio firmware) is often a giveaway. An unusually good one might show a genuine Samsung baseband version, but if it shows something else (e.g. a MediaTek baseband) then that would show that it was a fake.
If you just bought it, an obvious question is whether you can return it: if it's genuine it is faulty, so you should be entitled to a replacement or refund. Of course if you bought it from a Chinese online market you will most likely be out of luck.